Nrf1 (NF-E2 related factor1) belongs to the Cap'n'Collar-basic leucine zipper proteins (CNC-bZIP). The CNC-bZIP family is identified by its homology region, named the CNC domain, immediately N-terminal to the bZIP domain . This family contains a basic domain interacting with sequence-specific DNA and a leucine zipper domain (bZIP) involved in protein-protein dimerization [2–4]. The members of Cap'n'Collar (CNC) family contain Nrf1, Nrf2, Nrf3, p45 NF-E2, Drosophila CNC protein, as well as C. elegans Skn-1 [5–9]. There are two known transcriptional roles of the CNC-bZIP family. First, they are involved in globin gene expression. Transcriptional control of the human beta-globin gene cluster is mediated by four DNase I hypersensitive sites (HS1–4) spanning approximately 6–22 kb upstream of the epsilon-globin gene [10–13]. A similar gene structure (HS-40) is also present in alpha-globin gene expression [10, 14, 15]. The locus control region (LCR) appears to be necessary for high protein level expression from the entire globin gene clusters. The LCRs contain a direct sequence repeats 5'-(A/G)TGA(C/G)TCAGC(A/G)-3', which is the binding site for CNC-bZIP transcription factors . Second, they play a role in the antioxidant response. The consensus antioxidant response element (ARE) core sequence 5'-TGA(C/G)NNNGC-3' shows remarkable similarity to the aforementioned binding sequence for CNC-bZIP proteins. This similarity has led to the proposal CNC-bZIP factors can regulate detoxification proteins expression through AREs [17–21], such as expression of human gamma-glutamylcysteine synthetases (GCS), glutathione S-transferases (GST), UDP-glucuronosyl transferase (UDP-GT), NADP (H) quinone oxidoreductase (NQOs), etc.
Like the other members of CNC-bZIP factors, Nrf1 can heterodimerize with small Maf proteins to bind the cis-elements more efficiently [16, 22]. The small Maf proteins, including MafK, MafG and MafF, are identified by their homology to the avian transforming retroviral oncogene, v-maf with lacking of transactivation domains [23–25]. Transcriptional repression by Maf-Maf homodimers, yet activation by Maf-Nrf heterodimers, is shown in LCR-regulated gene regions . To further explore other factor(s) in cells related to Nrf1, we performed a yeast two-hybrid screening assay using the CNC-bZIP region of Nrf1 as a bait to identify any Nrf1-interacting proteins. In this study, cDNAs encoding residues 126–475 of MCRS2 (or residues 185–534 of p78/MCRS1, or residues 113–462 of MSP58 [26–28] were isolated from the HeLa cell cDNA library. In this paper, we characterized the physical and functional interactions between Nrf1 and MCRS2.
Some functions of MCRS1 and its splice variant-MSP58 and MCRS2 have been reported before, such as transforming, nucleolar sequestration, ribosomal gene regulation, and telomerase inhibition activities [28–32]. For example, TOJ3, a protein with high structural similarity to MCRS1, display transformation activity . In addition, the tumor suppressor gene PTEN suppresses the transformation activity of MCRS1 . It was reported MSP58 can relieve the repressor activity of Daxx, an adaptor protein that links Fas signaling to the c-Jun NH2-terminal kinase pathway by a nucleolar sequestration mechanism . Further, MCRS2 is also shown to be involved in telomere shortening by interacting with telomerase . Although these studies illustrates several functions of MCRS1/p78 and its splice variant, the roles of MCRS2 on Nrf1 transactivation activity have not been previously identified. Here we will show MCRS2 can function as a repressor on the Nrf1-mediated transactivation.